1. The Field of the Invention
The present invention relates generally to switching circuits. More specifically, the present invention relates to switching circuits that have controlled switching of the switching transistor for reduced Electro-Magnetic Interference (EMI), and that precharge the switching transistor for reduced switching delay.
2. Background and Relevant Art
Electronic circuitry has contributed enormously to the advancement of civilization. One of the fundamental building blocks of electronic circuitry is the switching circuit, in which a node of the circuitry is switched from one voltage to another in response to a switch control signal.
Designers of switching circuits take various constraints into account when conceiving such circuits. One of these constraints is the electromagnetic compatibility (or EMC) of the silicon chip they are designing. While operating, the chip should not radiate excessive electromagnetic energy in order not to interfere with the proper operation of other surrounding circuits. Such interference is often termed Electro-Magnetic Interference (EMI).
Electro-Magnetic energy will radiate to some extent every time a time-dependent voltage signal is present on chip. In general, the faster that voltage signal varies, the more electro-magnetic energy is radiated. Since switching circuits by their very nature involve the transition of a voltage signal from one voltage to another, switching circuits also involve rapid changes in current to sustain the voltage signal transition at a particular node. These rapid changes in current cause the switching circuits to radiate EMI. EMI may be of particular concern in Pulse Width Modulation (PWM) drivers, which often generate sharp transition edges.
One conventional solution to limit EMI is to limit the slope of the transition edge when switching the switching circuit. An example of such a conventional switching circuit is illustrated in FIG. 7 as switching circuit 700. The slope of the transition edge is controlled by the current sources IQ1 and IQ2 and the capacitor C1. Accordingly, this switching circuit emits less EMI than switching circuits that do not implement slope control.
However, a main disadvantage of that switching circuit is that it has a very long delay between the command to perform a switch to the actual start of the switch. The turn-on delay is caused by the necessity for the gate to source voltage of switch transistor M1 to reach the threshold voltage of the switch transistor M1 before the switch transistor M1 starts conducting. The turn-off delay is caused by the necessity for the gate to source voltage of switch transistor M1 to drop before exiting the linear region. FIG. 8 illustrates a timing signal diagram 800 of the transition control signal VON and the switch voltage VDS across the switch transistor M1. Note the turn on delay t1 and the turn off delay t2.
Some conventional switching circuits reduce switching delay by pre-charging the gate terminal of the switch transistor prior to an off-on transition, and pre-discharging the gate terminal of the switch transistor prior to the on-off transition. However, the gate terminal should not be pre-charged to a point where the switch transistor conducts significant current. Likewise, the gate terminal should not be pre-discharged to a point where the switch transistor reduces significantly the amount of current conducted. Otherwise, the output voltage would be altered despite not being in a transition. To avoid this, a significant safety factor is built into the pre-charge and pre-discharge. This safety factor may be quite large due to the variability in transistor performance. This large safety factor often prevents much of any possible pre-charge or pre-discharge. Accordingly, such conventional circuits may still have significant switching delay.
Furthermore, this and other conventional circuits may use a closed loop to pre-charge and pre-discharge. Closed loops can be quite unstable if not designed properly. Accordingly, significant time is often expended in designing such closed loop pre-charge switching circuits.
What would thereby be advantageous are switching circuits that have slope control during transitions to reduce EMI, that perform pre-charging to reduce switching delay, and that do not need significant design and/or testing to ensure stability during operation.